This invention relates to an improved process for calcining calcium carbonate (CaCO.sub.3) in a rotary kiln to obtain calcium oxide (CaO).
In the manufacture of paper by the Kraft and other alkaline pulping processes wood chips are cooked with a solution of sodium hydroxide, sodium sulphide, and other chemicals depending on the grade and kinds of pulp desired and the spent liquor after it has been separated from the pulp is processed to recover values therein. At some stage of the process the spent materials being treated are causticized with lime usually with resultant recipitation of calcium carbonate.
In order to improve the economics of the process the calcium carbonate is recovered and is subsequently calcined in a kiln to produce calcium oxide which may then be reused to causticize additional spent liquor. Naturally occurring limestone is also calcined to produce calcium oxide for a variety of known commercial uses.
There are two types of calcination kilns in general use. In the vertical shaft kiln coarse lime rock is mixed with solid fuel or, layered alternately and calcined to produce lime. Fine sized stone and fuel cannot be used since they would create too much resistance to the upward flow of combustion and reaction gas (CO.sub.2). The relative position of the limestone and fuel does not change during the descent of the charge in the kiln thus permitting a variety of solid fuels. Coal as well as wood has been used as fuel in the vertical shaft kilns. The vertical kiln is not suitable for the calcination of lime mud produced during causticization of spent materials in the pulping process due to the fineness of the mud.
The rotary kiln is especially suited for calcining finely powdered CaCO.sub.3 such as lime mud from the causticizing process. Hitherto the fuel used to supply the heat required to convert the mud to CaO has been exclusively natural gas, oil or powdered coal fired from the hot product discharge end of the rotary kiln.
U.S. Pat. No. 1,323,294 to R. W. Lesley relates to the use of low grade coal, oil shale or lignite in a cement calcination process employing a rotary kiln. The low grade material is gasified externally in separate retorts to obtain fuel gas and chemical values. The evolved gases are used to supplement or reduce the fuel conventionally fired from the product discharge end of the kiln. The gasification residue is mixed with the raw cement clinker. The fine carbonaceous part of the residue which has a heating value is burnt during calcination.
U.S. Pat. No. 4,022,629 to Garrett et al is directed to the manufacture of cement clinker in a rotary kiln. Combustion of a residual fuel is effected directly in the bed of the kiln to reduce the normal fuel requirement. Representative types of residual fuels are coke, coal, char, coal gasification residue or mixtures thereof. The chosen fuel must have a high heat value (over 10,000 BTU/lb), a maximum particle size of less than one quarter of an inch, (preferably between 8 and 325 U.S. screen mesh size) and a high ash and sulphur content.
Neither the ash nor the sulphur content of the above fuels can be tolerated in a kraft mill lime kiln.
I have discovered biomass material to be an inexpensive yet suitable fuel in the calcination of lime mud originating from the causticizing operations of an alkaline wood-pulping process such as kraft or soda.
The biomass material may comprise wood residues incidental to the wood chip production at a pulp manufacturing facility, or other woody residues, hog fuel, bark, sludges, agricultural residues and the like, provided it does not contain large amounts of sulfur or other elements which would adversely affect the use of the calcium oxide in the causticizing operation for which it is intended.
The use of a low grade biomass fuel when combusted directly in the bed of a rotary kiln provides a substantial portion of the required process heat thereby reducing the need for more expensive natural gas, oil or powdered coal fired in the hot produce discharge end of the kiln.
It is a primary object of the invention to provide an economical method of calcining lime mud or lime stone in a rotary kiln using a low grade material as the fuel to provide a substantial portion of the heat required for calcination. A second object is to provide a low grade, relatively sulphur-free, fuel material which will not affect the normal operation of the kiln. Another object is to recover the lime product from the kiln and utilize it to causticize a wood pulping liquor. A further object is to recover the heat values of any char and/or volatile organic material escaping with the combustion gases from the kiln for further combustion in the kiln. Other objects will be apparent to those skilled in the art.